1. Field of the Invention
The present invention relates to a data receiving device, and more specifically, to a data receiving device that reduces influences of a signal attenuation and propagation skew generated when high-speed digital data is transmitted through a differential cable.
2. Description of the Related Art
A digital data transmission system that propagates, as a differential signal, a clock or NRZ data through paired conductor such as a twisted-pair cable has many advantages: it has a feature of being invulnerable to fluctuation in potential difference of a transceiver, it can eliminate external noise by means of common-mode voltage elimination function of a differential receiving circuit; and it can reduce unnecessary radiation. Therefore, the digital data transmission system is widely used for high-speed data transmission in the medium range (about 10 to 100 meters).
In the conductor, there inevitably occurs a limited signal attenuation per unit of transmission length. This attenuation becomes prominent at high frequency due to skin effect. Accordingly, a long transmission distance and high transmission data rate conflict with each other. In order to eliminate the conflict, a technique in which an equalizer is provided on the receiving side to compensate an attenuation in a transmission path to allow a receiver to reproduce the signal that seems not to have experienced an attenuation of the conductor is available. In this case, since attenuation amount changes depending on the conductor serving as a transmission path, an active circuit with variable frequency characteristics is used for wider application (refer, for example, to Jpn. Pat. No. 2830087). Further, a technique of automatically adjusting the characteristics is available (refer, for example, to Jpn. Pat. Appln. Laid-Open Publication No. 11-122041).
However, the above adaptive equalizer has been designed on the assumption that the NRZ data probabilistically transits with frequency. Therefore, reception fault tends to occur for specific transition pattern such as long string of 0s, which may often appear in actual data.
There exists another factor that contributes to the conflict between a transmission distance and transmission data rate in the transmission of differential signal through the paired conductor such as a twisted-pair cable.
Since there is a difference in physical length or dielectric constant of coating in the conductor pair by necessity, a limited difference is made also in propagation delay time. In the differential signal that has been propagated through the conductor pair, a phenomenon in which positive and negative signals reach the receiving end with time difference is generated. This phenomenon is referred to “skew”, and the skew causes inter-symbol interference in which the transmission signal misaligned in time is superposed on a receiving end differential signal. In the case of widely used twisted-pair cables, it is not uncommon that skew of up to 10 ps is observed per 1 m. When the skew exceeds ¼ of NRZ bit period, proper data reception cannot be realized simply by detecting zero cross of the differential signal with a simple comparator. Therefore, the distance at which 2 Gbps signals having a bit period of 500 ps can be transmitted using a differential transfer is limited to about 12.5 m.
For example, FIG. 1 shows attenuation characteristics of a typical cable. In FIG. 1, transmission signals of image data are used. Although attenuation characteristics differ depending on formats such as UXGA, SXGA, XGA, amplitude attenuations of any of signals become large with increasing cable length. FIG. 2 shows skew characteristics between a typical cable pair. It is said that twist tension control or adjustment at the time of assembling the connector can reduce the skew to about ⅓. However, the skew increases with increasing cable length.
In order to overcome the above constraints and realize an effective transmission, it is necessary to adjust, for each cable pair, propagation delay of the positive and negative signal lines of the twisted-pair cable. As the circuit that removes inter-symbol interference caused by skew, the equalization technique using a transversal filter related to an optical disk data reproduction disclosed in Jpn. Pat. No. 3208865 or the equalization technique using a multi-threshold comparator related to optical fiber communications disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 11-243428 can be employed.
However, even when the above techniques are employed, in order to manufacture the receiving device capable of widely coping with the skew amount that varies depending on the cable, delicate and complicated control needs to be performed so that additional weighting factor of the transversal filter or threshold of the multi-threshold comparator can be properly equalized, in consideration of inter-symbol interference caused by the skew amount and high frequency attenuation due to the aforementioned skin effect. Further, the transversal filter or multi-threshold comparator is a high-speed circuit that operates at an NRZ transmission speed. Therefore, in order to realize a high-speed data transmission, the transversal filter or multi-threshold comparator must be operated at high-speed and with high accuracy, resulting in large power consumption of the receiving device.